Electrochemical sensors are widely used to determine electroactive chemical species in liquid, gas and vapor phases. Electrochemical sensors or cells in which measurable current flows can be conveniently classified as galvanic when operated to produce electrical energy and electrolytic when operated at a constant potential via consumption of electrical energy from an external source. Many electrochemical sensors can be operated in either a galvanic or an electrolytic mode. A comprehensive discussion of electrochemical gas sensors is also provided in Cao, Z. and Stetter, J. R., "The Properties and Applications of Amperometric Gas Sensors," Electroanalysis, 4(3), 253 (1992), the disclosure of which is incorporated herein by reference.
In a typical electrochemical sensor, the chemical entity to be measured (the "analyte") typically diffuses from the test environment into the sensor housing through a porous or permeable membrane (through which the analyte is mobile, but through which the electrolyte is not mobile) to a working electrode (sometimes called a sensing electrode) wherein the analyte chemically reacts. A complementary chemical reaction occurs at a second electrode in the sensor housing known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte at the working and counter electrodes.
In general, the electrodes of an electrochemical sensor provide a surface at which an oxidation or a reduction reaction occurs (that is, an electrochemically active surface) to provide a mechanism whereby the ionic conduction of an electrolyte solution in contact with the electrodes is coupled with the electron conduction of each electrode to provide a complete circuit for a current. By definition, the electrode at which an oxidation occurs is the anode, while the electrode at which the "complimentary" reduction occurs is the cathode.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte over the concentration range of interest.
As discussed above, the electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The primary functions of the electrolyte are: (1) to efficiently carry the ionic current; (2) to solubilize the analyte; and (3) to support both the counter and the working electrode reactions. The primary criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
In current electrochemical sensors, every effort is generally made to achieve a quick response time. The response time is typically defined as the amount of time an electrochemical sensor must be exposed to a particular concentration of an analyte before an accurate measurement of that concentration can be made by the electrochemical sensor. The response of a living organism (for example, the human body) to certain chemicals, however, is often a very complicated function of time of exposure, the concentration of the chemical in the environment, and other factors. The uptake of or concentration of a chemical (or the concentration of a reaction product resulting from exposure to such a toxic chemical) in the human body is often expressed as a dose-response curve which sets forth such concentration as a function of time for specific environmental exposure concentrations. Over a broad range of environmental concentrations, a substantial amount of exposure time may be required before any adverse effect is experienced. In general, the higher the environmental concentration of a chemical, the shorter the exposure time required to experience adverse effects. Current electrochemical sensors cannot model the dose-response behavior of a living organism upon exposure to a chemical.
It is very desirable to develop electrochemical sensors capable of providing a response which approximates the dose-response behavior of a living organism upon exposure to a chemical, and, particularly a toxic chemical.